[Update]
iTunes 9.2 is now officially release for Mac and Windows. Get it here.

Apple released iOS 4 GM for developers, however you can have it too! iOS 4 is said to bring several new features such as multitasking (unlike desktop multitasking), better security, iBooks and more API for app developers.

iTunes 9.2 and iOS 4 will be officially released on 21st June and free for all iPhone, iPod Touch 2G and later.

Due to its new updates and enhancement, iOS 4 need iTunes 9.2 Beta to be able to sync with your device. Thus, you need be clear that you won’t be able to resync with any lower version of iTunes. To date, the Beta version is available only for Mac OSX.

Worry not, after the upgrade, you can still downgrade and revert back current version. To start, download all the necessay files and software namely iTunes 9.2 Beta, iOS 4 GM image file and Redsn0w 0.9.5B5.

1. Grab a copy of iTunes 9.2 Beta and iTunes 9.1.1, incase for downgrading.
2. A copy of iOS 4 GM according to your device. Or in this case, I’m using iOS 4 GM for Ipod Touch 2G, click here.
3. And a copy of Redsn0w 0.9.5B5 to for jailbreaking.

LETS START

Next, follow all the instructions, step by step:

1. Firstly, backup your current devices.
2. Next, install the iTunes 9.2 Beta.
3. Then, Alt+Click (Mac OSX only) on the Restore button, and browse for the downloaded iOS 4 GM, according to your device and the iPhone/iPod version. Let it restore. This will remove all files and apps.
4. Now, we’re going to jailbreak the device using Redsn0w 0.9.5B5.
5. Run Redsn0w 0.9.5B5 then click Browse and select the same version of IPSW image of iOS 4 GM that you used on yor device.
6. Follow the instruction from Redsn0w and you should be able to jailbreak your devices. Make sure you selected Cydia.
7. Now, after the jailbreak, we need to update Cydia.
8. Open Cydia and let it updates itself. Wait and be patience. Then, navigate to Manage > Sources and select Edit.
9. Add these two repositories.
   - http://apt.saurik.com/cydia-3.7
   - http://cydia.hackulo.us
10. Then let the Cydia updatea and restart itself.
11. Next, we’re going to install AppSyc to enable us to resync with our jailbreak apps.
12. Navigate to Search in Cydia, and find AppSync and install. Restart.
13. Once done with upgrading to iOS 4, resync with your backup.
14. Now you’re done!

Now, you can start testing your experience on iOS 4 GM, and bare in mind, this is not the final release of iOS, thus things might not working as what have expected. Incase you decided to downgrade back to previous version, read the below instructions.

DOWNGRADING

1. Download iTunes 9.1.1
2. Download iPhone OS 3.1.2 according to your device.
3. Download Spirit for jailbreaking.
4. Firstly, backup the device.
5. Then, install iTunes 9.1.1 to downgrade from iTunes 9.2 Beta.
6. Get into DFU mode and not recovery mode.
   • Connect your iPhone to your computer.
   • Turn iPhone off.
   • Hold power and home together for 10 seconds (exactly).
   • Release power but keep holding home until the computers beeps as a USB device is recognized.
   • In seconds, iTunes should detect your iPhone ready for restoration.
   • If the Restore Logo is present on the screen, you are in Recovery Mode, not DFU
7. Hold Alt+Click on the Restore button, and browse and restore using the 3.1.3 IPSW you downloaded.
8. If error occur, read through this page to exit the Recovery Mode.
9. Run Spirit JB tool and press jailbreak. things should done in seconds.
10. install AppSync using by adding http://cydia.hackulo.us repository in the Cydia sources, and search for it.
11. Now you’re ready to sync with your iTunes.

HAPPY UPGRADING AND JAILBREAKING!